kingst-la2016: move acquisition abort from api.c to protocol.c

kingst-la2016: move acquisition abort from api.c to protocol.c

There was the abort_acquisition() routine (whithout a prefix) and the
la2016_abort_acquisition() routine (with a prefix). Fold the termination
of USB reception into the drivers's SR_PRIV routine which resides in
protocol.c, which eliminates the misplaced api.c implementation. Change
a few more receive callback related routines from SR_PRIV to static now
that their calls are intra compile unit.